Automated material handling systems (AMHS) are widely used in semiconductor fabrication facilities (FABs) to automatically handle and transport groups or lots of wafers between various processing machines (i.e. semiconductor tools) used in chip fabrication. The AMHS is computer controlled for handling the staging of wafers for processing and the flow of wafer traffic in the FAB. In addition, an AMHS may also be used to automatically handle and transport photomask reticles between various semiconductor tools during the fabrication process.
The AMHS typically includes numerous types of automated and manual mechanisms for moving and transporting the carriers for storing wafers or reticles throughout the FAB during the fabrication process. This can include, for example, automatic guided vehicles (AGVs), personal guided vehicles (PGVs), rail guided vehicles (RGVs), overhead shuttles (OHSs), and overhead hoist transports (OHTs).
Among the foregoing transport mechanisms of the AMHS, OHTs are commonly used to transport the wafer carriers or retile carriers from the load port of one semiconductor tool to the load port of the next semiconductor tool in the processing sequence. An OHT system includes at least one vehicle that travels on an overhead monorail of the AMHS. The vehicle on-board hoist is operable to raise and lower the wafer carriers or reticle carriers, thereby allowing the vehicle to deposit and retrieve the wafer carriers or reticle carriers from the load ports of semiconductor tools positioned along and on the floor beneath the overhead rail.
Although existing OHT systems for transporting wafers or reticles in a FAB during the fabrication process have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.